1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as TFT) in which a crystalline semiconductor film is used for a semiconductor layer, and to a semiconductor device in which the TFT is used as a switching element in a driver circuit or a pixel portion (particularly, liquid crystal display device). In particular, the present invention relates to a TFT and a semiconductor device, in which a crystalline semiconductor film formed by using a catalytic element that promotes crystallization is used.
2. Description of the Related Art
There have been active studies on a liquid crystal display device (also referred to as liquid crystal panel) in which a TFT is used as a switching element in a pixel portion, in particular, a liquid crystal display device in which a driver circuit and a pixel portion are formed on the same substrate. The TFT uses a crystalline semiconductor film (typically, a crystalline silicon film) that has high electric field effect mobility and fast movement of carriers to enable high speed operation, instead of a TFT in which an amorphous semiconductor film (typically, an amorphous silicon film) is used. The above-described liquid crystal display devices are being sold in actuality.
A method of irradiating laser light, a thermal crystallization method by heating, a method using a catalytic element, and the like can be given as methods for manufacturing a crystalline silicon film.
In the case of using a crystalline semiconductor film formed by using a catalytic element for a semiconductor layer of a TFT, the catalytic element used for crystallization of a semiconductor film is moved to a gettering region from an element region (particularly, a channel forming region) of the TFT in order to improve characteristics of a semiconductor device. Thus, after an element which has a gettering action is added to form the gettering region, heat treatment is conducted.
As gettering processes, there are given and used: a method in which an impurity element that imparts one conductivity (n-type) to a semiconductor layer and has a gettering action is added to a source region or a drain region after a crystalline semiconductor film is formed, and heat treatment is conducted to perform activation of the impurity element and gettering of a catalytic element in the same step; a method in which an impurity element which has a gettering action is added to a region except an element region of a semiconductor film to successively form a gettering region after a crystalline semiconductor film is formed, and heat treatment is conducted to perform gettering of a catalytic element; and other methods.
In case of the former method, an n-type impurity element (typically, phosphorous) at high concentration is uniformly added to the gettering region (that later becomes a source region or a drain region) of each of an n-channel TFT and a p-channel TFT. Therefore, the source region or the drain region of the p-channel TFT has to be doped with a p-type impurity element at a concentration twice to three times as high as that of the n-type impurity element in order to reverse n-type to p-type. Thus, a process for adding impurities takes a long time, and there is a problem of throughput. Further, the excessive addition of ions that become acceptors causes a problem of manufacturing cost and a problem that crystallinity of a semiconductor film is broken and it is difficult for the semiconductor film to be recrystallized to raise a resistance and lead to reduction of an on current.
Therefore, the inventors of the present invention performed evaluation on a gettering efficiency of several samples shown in Table 1 below in order to find a way of sufficiently gettering a catalytic element without excessively adding an impurity element to a source region or a drain region that becomes a gettering region in a p-channel TFT. FIGS. 2 to 5 each show a state in which etch pits are observed. In FIGS. 2 to 5 each, the sizes of channel forming regions are 5, 10, 15, 20, 30, and 50 xcexcm from the upper portion in the figure. The evaluation is also performed on the size of the channel forming region in which gettering can be sufficiently performed.
The present inventors selectively remove NiSix in order to find the gettering efficiency after the heat treatment for gettering, and the gettering efficiency is evaluated based on the number of holes that occur after the removal. The hole is called an etch pit. In the case where the etch pit is not observed in the channel forming region, it is evaluated that the catalytic element that remained in the channel forming region can be moved to the gettering region.
It is considered that the catalytic element (Ni) is bonded to Si to become NiSix in a process that the catalytic element moves from the channel forming region to the gettering region. A silicon oxide film is removed with a mixed solution containing 7.13% ammonium bifluoride (NH4HF2) and 15.4% ammonium fluoride (NH4F) (made by Stella Chemifa Corp., trade name: LAL500), and a sample substrate is immersed in an aqueous solution of chemicals mixed with a volume ratio of HF (concentration of 50%): H2O2 (concentration of 33%): H2O=45:72:4500 (hereinafter referred to as FPM) for 40 minutes, whereby NiSix can be selectively removed. The holes are generated in the portions where NiSix is removed. The holes formed after removing NiSix can be observed as black points of the samples in a transmission mode of a light microscope. Note that the black point is referred to as the etch pit in this specification.
Even a base insulating film (silicon oxide film), which is formed at the substrate side of the silicon film, is removed due to a processing time of etching and an aqueous solution of chemicals for processing, and thus, the etch pit becomes somewhat larger than the original size of NiSix. However, since precipitated NiSix is removed, the etch pit is considered to have substantially the size of NiSix.
Here, the present inventors direct their attention to the fact that there is a large difference in the state after etching among samples A, B, and D which are judged form that the gettering efficiency is sufficiently high (the etch pit is not seen even with the size of the channel forming region of 15 ì m). In the sample B, the etch pit with a large hole-shape can be observed while in the sample A and the sample D, the etch pit can not be observed. What is seen as a dot shape in the sample D is seemed to be a flaw that is generated on the surface of the silicon film at the time of adding the impurity element. The present inventors surmised that it is difficult to occur the precipitation of NiSix in the sample A and the sample D, in other words, Ni exists (is solubilized) as an element in silicon lattices. Further, as to the sample D, they considered that a p-type impurity element is added at high concentration and strong p-type conductivity is imparted, whereby there is reached the state that boron (B) and nickel (Ni) are easily bonded to each other and NiSix becomes difficult to be generated. Further, NiSix is easy to precipitate when phosphorous (P) and argon (Ar) exist in the gettering region.
There has been a problem that an off current suddenly rises in a TFT for which a crystalline semiconductor film formed by using a catalytic element is used. The inventors of the present invention consider that NiSix precipitates in a defect of a semiconductor layer in the crystalline semiconductor film formed by using the catalytic element, in particular, NiSix precipitates in a junction portion between a channel forming region and a source region or drain region, as a result, off current suddenly rises.
Then, the present inventors consider that, if only P is added to a gettering region of an n-channel TFT; B and Ar are added to a gettering region of a p-channel TFT; and then gettering is performed, a catalytic element (Ni) can be gettered from a region used as an element in a state that the catalytic element (nickel) singly exists (is solubilized) in silicon lattices with suppressing precipitation of NiSix.
The present invention has been made in view of the above, and has its object to realize a TFT which has good characteristics by gettering a catalytic element with suppressing precipitation of NiSix in the entire region that becomes elements to form a high-quality crystalline semiconductor layer, and manufacturing a TFT using the above-described crystalline semiconductor layer.
Another object of the present invention is to realize a semiconductor device with good characteristics which includes a circuit manufactured by using the above-described TFT.
In the present invention, a gettering region (a region to which phosphorous (P) is added) of a semiconductor layer, which is a region that later becomes an n-channel TFT, is not doped with a rare gas element such as argon (Ar) but with only an n-type impurity element with a gettering action, typically phosphorous (P). A gettering region (a region to which boron (B) is added) of a region that becomes a p-channel TFT is doped with a rare gas element such as argon (Ar) which has an action of enhancing a gettering effect, in addition to a p-type impurity element which has a gettering action, typically boron (B). Then, gettering is performed. Thus, gettering of a catalytic element (nickel) is performed so as not to precipitate NiSix and so as to obtain a solubilized state that nickel (Ni) exists as an element in a lattice of silicon. The gettering of a catalytic element is performed so that precipitation of little NiSix is generated in a source region or a drain region of the semiconductor layer, in particular, in a junction portion between a channel forming region and the source region or drain region. Accordingly, the semiconductor device according to the present invention is provided. The concentration of the n-type impurity element in the source region or the drain region of the n-channel TFT is 5xc3x971019 to 5xc3x971021/cm3. Further, the concentration of the p-type impurity element in the source region or the drain region of the p-channel TFT is 1xc3x971019 to 5xc3x971021/cm3. Moreover, the concentration of the rare gas element in the source region or the drain region of the p-channel TFT is 1xc3x971019 to 1xc3x971022/cm3.
According to the present invention, the impurity element to be added to the gettering region is changed in accordance with the conductivity of the TFT, whereby the catalytic element can be sufficiently moved to the gettering region from the semiconductor layer (the channel forming region). Thus, the precipitation of NiSix, considered to adversely affect characteristics of the TFT, can be suppressed, and throughput can be improved. Further, a semiconductor device in which reliability is improved can be realized.